1. Field of Invention
This invention relates to devices which measure the displacement of an object sliding across a surface of snow or ice such as a ski, snowboard, sled, luge or ice skate.
2. Prior Art
Numerous methods have been proposed to measure the speed of a ski. Originally these methods used some form of mechanical contact measurement such as a wheel or roller which was attached to the tail of a ski via a spring loaded arm. U.S. Pat. No. 4,136,451 is such a device which consists of a wheel which rolls on the snow surface. The wheel is attached to a spring loaded arm which pivots about an axis which is perpendicular to the axis of the ski but in the plane of the ski. This allows the wheel to remain in contact with the snow if the tail of the ski rises up a certain amount.
U.S. Pat. No. 4,860,585 describes a similar device, also using a mechanical cog type roller to detect speed. This device is also attached to the tail of the ski via a pivot perpendicular to the axis of the ski. This device has the advantage that the roller extends the width of the ski which improves the contact to a certain extent when the ski is at lower edge angles.
U.S. Pat. No. 4,864,860 describes a device which uses a roller mounted to the tail of the ski to drive a generator. The generator is held against the snow by an arm which rotates about a pivot perpendicular to the axis of the ski like the two previously mentioned references.
U.S. Pat. No. 6,216,536 describes a similar device that pivots both about an axis perpendicular to the longitudinal axis of the ski in the plane of the ski as well as a second axis normal to the plane of the ski. By adding the second axis, the device is able to determine side slip behind the tail of the ski as well as forward speed under some circumstances.
The four aforementioned references share a number of disadvantages. First, the tail of a ski is a very unwieldy place to have something that rises above the surface of the ski. Ski tails are often crossed either unintentionally while skiing or intentionally when using the skating technique, which is very common when starting a run, making a turn at slower speeds, or crossing a flat section of a slope. Each of the above devices would prevent a skier from crossing the tails of the skis thus provoking a dangerous situation or preventing the skis from being used in one of the required modes of skiing.
In addition to being dangerous and limiting certain ski techniques the tail of a ski as a mounting location has another serious disadvantage as it is subject to very high vibration loads. Ski Rossignol has measured the vibration on the tail of a ski at up to 1000 g. With this type of force, it would be very hard to keep a tail-mounted device against the snow. If a spring strong enough to keep the device down were used, it would bend the tail of the ski up off the snow and interfere with the performance of the skis.
While the first three references have no means of measuring sideways slip (a very important characteristic of ski technique) U.S. Pat. No. 6,216,536 does have a pivot normal to the plane of the ski which allows the measurement of slippage under certain very constrained conditions. However, high edge angles (where most slip occurs), high vibrations (unavoidable on the tail of a ski), and the ski bent into an arc by carving the ski on its edge, would all prevent this device from accurately measuring slippage when mounted behind the tail of the ski. Additionally, locating the measurement device behind the tail of the ski distorts the measurement of slippage, as ski tails slide much more than the center or tip of the ski.
Another category of devices use non-contact means of determining speed. Most notably DE 195 24 842 talks of using Doppler technique, laser anemometer, or an optical signal correlation technique such as those provided by CORRESYS-DATRON Sensorsysteme GmBH and U.S. patent application Ser. No. 10/346,713 from this applicant uses optical navigation technology to optically determine the displacement of spatial patterns using spatial pattern recognition. Both of these references show embodiments where the non-contact means of determining speed is located inside the body of the ski. While both these references note that it is possible to mount the sensors somewhere other than inside the body of the ski, neither describe any mechanical apparatus for doing so.
Mounting a speed detection system inside a ski has a number of serious disadvantages, notably, the ski structure must be modified, which requires the skis to be built at the factory in such a way that they are enabled for the device. In addition, changing the inner structure of the ski changes the skis characteristics as well as it would require ski manufacturers to retool their assembly lines. Lastly, a device built into the ski, could not be used with existing skis, substantially reducing the size of the market.